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receiver. The contents of the retort are now cooled to
about 8oc and a further 50 grams of oxalic acid added. The
reaction recommences on heating with the formation of aqueous
formic acid, which becomes more concentrated with each fresh
addition of oxalic acid until the distillate eventually contains 56
per cent, of acid. The other portions of oxalic acid are added
in the same way. In order to regain the formic acid which
remains as monoformin in the retort, the contents are trans-
ferred to a round flask, diluted with about 250 c.c. of water
and distilled in steam, until the distillate has only a faintly acid
reaction (about 250 c.c.).

Distillation in Steam.—The apparatus for distilling in
steam is shown in Fig. 68.    A large flask, or, preferably, a i gallon

FIG. 68.
tin is closed by a double bored cork. A safety-tube passes
through one hole, and a bent tube which terminates below the
cork passes through the second hole, and is attached by rubber
tubing to the inlet-tube of the distilling flask (i litre). The
flask is sloped to prevent the contents being splashed over
into the condenser. It is heated on the sand-bath or asbestos
board to boiling, and steam passed in. The united distillates
are poured into a basin and neutralised by adding lead car-
bonate until, on heating, no further effervescence occurs. The
liquid is now left for a moment to settle, and the clear solution
decanted, whilst hot, through a fluted filter. The residue in the